In recent years, in response to social requirements and movements due to energy and environmental problems, fuel cells capable of operating at room temperature and having high power density have attracted attention as power supplies for electric vehicles and fixed power supplies. A fuel cell, which produces water in the electrode reaction in principle, is a clean power generation system having few adverse effects on the global environment. Fuel cells include polymer electrolyte fuel cells (PEFCs), phosphoric acid fuel cells (PAFCs), alkaline fuel cells (AFCs), solid oxide fuel cells (SOFCs), molten carbonate fuel cells (MCFCs), and the like. The polymer electrolyte fuel cells hold promise as power sources for electric vehicles because the polymer electrolyte fuel cells can operate at comparatively low temperature and have high power density.
A general polymer electrolyte fuel cell has a structure in which a membrane electrode assembly (hereinafter, also just referred to MEA) is sandwiched by separators. The MEA includes a solid electrolyte membrane sandwiched by a pair of electrode catalyst layers and if necessary, by gas diffusion layers.
Each of the electrode catalyst layers is a porous substance made of a mixture of a polymer electrolyte and an electrode catalyst including catalyst particles supported on an electroconductive support. Each of the gas diffusion layers includes a water-repellent carbon layer formed on a surface of a gas diffusion substrate such as carbon cloth. The water-repellent carbon layer is composed of carbon particles, a water repellant, and the like.
In the polymer electrolyte fuel cell, the following electrochemical reaction proceeds. First, hydrogen contained in fuel supplied to the anode-side electrode catalyst layer is oxidized by the electrode catalyst into protons and electrons as shown by the following Formula (1). Next, the generated protons pass through the polymer electrolyte contained in the anode-side electrode catalyst layer and the electrolyte membrane in contact with the anode-side electrode catalyst layer, and reach the cathode-side electrode catalyst layer. The electrons produced in the anode-side electrode catalyst layer pass through the electroconductive support constituting the anode-side electrode catalyst layer, the gas diffusion layer in contact with a surface of the anode-side electrode catalyst layer opposite to the solid electrolyte membrane, a separator, and an external circuit to reach the cathode-side electrode catalyst layer. The protons and electrons having reached the cathode-side electrode catalyst layer react with oxygen contained in oxidant gas supplied to the cathode side through the electrode catalyst, thus producing water. In the fuel cell, electricity can be extracted to the outside by the aforementioned electrochemical reaction.
(Chemical Formula 1)Anode: H2→2H++2e−  (1)Cathode: O2+4H++4e−2→H2O  (2)
The polymer electrolyte contained in the fuel cell does not provide high protonic conductivity when the polymer electrolyte is not wet. It is therefore necessary to humidify the reaction gas supplied to the polymer electrolyte fuel cell using an auxiliary such as a gas humidifier. However, humidifying the fuel cell using such an auxiliary causes complication and enlargement of an entire fuel cell system, reduction in power generation efficiency, and the like.
Japanese Patent Unexamined Publication No. 2002-203569 discloses an electrode catalyst layer with a gas-phase side surface covered with a water-repellent layer having a reaction gas permeability.